1. Field of the Invention
The present invention relates to a method and an apparatus for acquiring magnetic resonance imaging (MRI) signals, and more particularly to a method and an apparatus for simultaneously acquiring multi-slice/slab MRI signals including applying at least one coherent refocusing gradient.
2. Background of the Invention
A magnetic resonance imaging (MRI) apparatus is configured to reconstruct MR images based on MR signals. An MRI apparatus typically applies a static magnetic field, a gradient magnetic field, and a radio frequency (RF) signal having a selected frequency to a subject to excite a selected nucleus type and then detects the MR signals responsively sent by the excited nucleus.
Referring to FIG. 1, in a conventional procedure of 2D spatial encoding, only one single slice of the subject can be processed at a time; that is, multi-slice images are acquired from multiple scans along a scan direction. Therefore, one image is obtained from one scan; N images are obtained from N times of scan. The time required for acquiring the images of all the slices can be calculated as Equation 1 (Eq. 1).
Time required for acquiring the images of all the slices=NEX×Npe×TR×Nslice,  (Eq. 1)
where NEX denotes the average number of repeated signaling of a single slice and Npe is the whole number of encoding. For the 2D MR image, Npe denotes the number of phase encoding Np (scan lines in the k-space), TR denotes the time required for acquiring a scan line in the k-space, and Nslice denotes the number of the slices. For example, if there are on the total 256 128×128 images to be acquired, then Nslice=256, NEX=1, Npe=128, TR=0.1 second, and the time required for acquiring the images of all the slices is about 54 minutes. This is indeed a time-consuming procedure.
Referring to FIG. 2, in a conventional procedure of 3D spatial encoding, in one scan only one single slab of the subject can be excited to give the images of all the slices. Also, the time required for acquiring the 3D MR image is calculated as Eqn. 1, but the whole number of encoding Npe=Np×NZ, wherein Np is the number of phase encoding and NZ is the number of phase encoding. Thus, it is apparent that more time is required for acquiring the 3D MR images.
Our previous application (US Publication No. 20090278538) has revealed a novel method and apparatus for simultaneously acquiring multiple slices/slabs in magnetic resonance system. The method comprises the following steps: applying one or more than one RF pulse 301, which carries at least two frequency components, and a slice/slab selection magnetic field gradient 104 so that at least two slices/slabs of the subject respectively corresponding to the at least two frequency components can be excited simultaneously; applying spatial encoding magnetic field gradients 102, 103; and applying a slice/slab separation magnetic field gradient 105 so as to separate the at least two slices/slabs. The method according to the previous invention can be used to acquire data for simultaneously reconstructing multiple slices/slabs.
However, the images acquired by the said method can be blurry. The level of blur is required to be set in a preset range so that sharp and acceptably clear images can be acquired. To mitigate the image blur problem, the thickness of a slice is limited from above. Otherwise, when the level of blur is higher than 3 pixels, the quality of the acquired MR images is poor.